CDMA communication system and its transmission power control method

ABSTRACT

An uplink channel transmission power control method is provided for a CDMA mobile communication system performing one way communication. A base station measures the received level of data transmitted from each mobile terminal at each channel, and generates a transmission power control signal of each uplink traffic channel. The generated transmission power control signals are multiplexed, and the multiplexed common transmission power control signal is transmitted to all mobile terminals by using the common channel shared by the mobile terminals. Each mobile terminal derives the transmission power control signal of the uplink traffic channel used by the terminal, from the received common transmission power control signal, and controls the transmission power of a data packet.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a code division multiple accessmobile communication system and its transmission power control method.More particularly, the present invention relates to a packetcommunication system and its transmission power control method usingreservation based access control.

[0002] In a CDMA method, a plurality of mobile terminals share the samefrequency band to communicate with a single base station. Therefore, forexample, if mobile terminals A and B transmit modulated signal waves tothe base station, the signal (not desired to be received) transmitted bythe mobile terminal B interferes with the signal (desired to bereceived) transmitted by the mobile terminal A, and the communication ofthe mobile terminal A with the base station is obstructed. The degree ofinterference depends on the received level of a signal (not desired tobe received) at the base station. If the degree of interference becomeslarge to some level or more, communication between the mobile terminaland base station becomes impossible.

[0003] If the transmission power of each mobile terminal can becontrolled to always limit the signal level received at the base stationto a minimum necessary reception power, it becomes possible to maximizethe number of channels capable of being communicated by the basestation. The more the transmission power shifts from the minimumnecessary reception power, the less the number of channels capable ofbeing communicated by the base station.

[0004] As transmission power control techniques of CDMA mobilecommunication, an IS-95 transmission power control method is knowndescribed in TIA/EIA/IS-95 which is a standard system of digital cellarphones adopted in North America. The IS-95 transmission power controlmethod will be described in the following.

[0005] Since two way communication is essential for cellar phones, apair of an uplink traffic channel and a downlink traffic channel is usedfor the communication between the base station and a mobile terminal.The uplink traffic channel is a channel for transmitting data from amobile terminal to the base station, and a downlink traffic channel is achannel for transmitting data from the base station to the mobileterminal.

[0006] The base station measures the reception power of data transmittedfrom each mobile terminal and generates a transmission power controlsignal in accordance with the measured reception power. If the receptionpower of data is larger than a target reception power, the base stationgenerates a transmission power control signal “1” for this mobilestation. Conversely if the reception power of data is smaller than thetarget reception power, the base station generates a transmission powercontrol signal “0” for this mobile station. The generated transmissionpower control signal is inserted into data to be transmitted from thebase station to a mobile terminal, and the transmission data with thetransmission power control signal is transmitted to the mobile terminal.The mobile terminal controls to reduce the transmission power if thereceived transmission power control signal is “1”, and to increase it if“0”.

[0007] This transmission power control will be described specificallywith reference to FIG. 12. Each mobile terminal 1 to n and the basestation communicate with each other by using a pair of an uplink trafficchannel and a downlink traffic channel. The upper row of each pairrepresents transmission data of the downlink traffic channel, and thelower row represents transmission data of the uplink traffic channel.The width of transmission data, particularly uplink transmission data,is drawn to correspond to a reception power of the uplink data at thebase station.

[0008] When the base station communicates with the mobile terminal 1, itinserts transmission power control signals 132 a, 132 b, 132 c, . . .into a downlink traffic channel 130 a to the mobile terminal 1. Themobile terminal 1 changes its transmission power of the uplinktransmission data in accordance with the transmission power controlsignal obtained from the received channel 130 a. As above, thetransmission power control of the mobile terminal 1 is performed byusing the downlink traffic channel 130 a. Similar transmission powercontrol is performed also for other mobile terminals 2 to n.

SUMMARY OF THE INVENTION

[0009] With advancement of mobile communication techniques, needs of notonly a voice communication function (cellar phone) but also a datacommunication function are becoming large.

[0010] For one way communication typical to data communication, CDMApacket communication systems have been proposed from the viewpoint ofefficiently using channels. One proposal of such CDMA packetcommunication systems is described in “Development on CDMA Packet MobileCommunication System” by Yano, Uta, Hasegawa, and Doi, CommunicationSociety Meeting, the Institute of Electronics, Information andCommunication Engineers, B-389 (1996).

[0011] Voice communication is two way communication using uplink anddownlink traffic channels, whereas data communication is one waycommunication using only one of uplink and downlink traffic channels. Insuch one way communication, a conventional transmission power controlmethod for cellar phones cannot be adopted because this method isestablished on the assumption that there is a pair of uplink anddownlink traffic channels.

[0012] If a paired downlink channel is provided only for thetransmission power control of the uplink traffic channel, one downlinktraffic channel is occupied by the transmission power control of onlythe uplink traffic channel. The use efficiency of traffic channels islowered.

[0013] To solve this problem, the invention provides a CDMA packet datacommunication system in which a base station controls the transmissionpower of each of a plurality of mobile terminals by using a singledownlink traffic channel common for all mobile stations.

[0014] The base station measures the received level of data transmittedfrom each mobile terminal at each channel, and generates a transmissionpower control signal of each channel in accordance with the measuredreception level. The generated transmission power control signals arecollected together into a format predetermined for the system, andtransmitted to all mobile terminals by using the common channel sharedby the mobile terminals.

[0015] Each mobile terminal derives the transmission power controlsignal of the uplink traffic channel used by the terminal, from thecollected transmission power control signals transmitted from the basestation, and transmits data at the transmission power changed inaccordance with the derived transmission power control signal.

[0016] These and other objects, features and advantages of the presentinvention will become more apparent in view of the following detaileddescription of the preferred embodiments in conjunction withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a diagram showing the structure of a mobilecommunication network.

[0018]FIG. 2 is a diagram illustrating a packet data communicationsystem using reservation based access control.

[0019]FIG. 3 is a diagram showing a first example of the structure of abase station embodying transmission power control of the presentinvention.

[0020]FIG. 4 is a diagram showing the structure of an answer packet.

[0021]FIG. 5 is a diagram showing the structure of a unit for measuringa received level of a traffic channel.

[0022]FIG. 6 is a diagram showing the structure of a unit for generatinga transmission power control signal of a traffic channel.

[0023]FIG. 7 is a diagram illustrating insertion of a transmission powercontrol signal between answer packets.

[0024]FIG. 8 is a diagram showing a first example of the structure of amobile terminal embodying the transmission power control of theinvention.

[0025]FIG. 9 is a diagram illustrating a transmission power controlstate of an uplink traffic channel realized by the operations of a basestation and mobile terminals according to the present invention.

[0026]FIG. 10 is a diagram showing a second example of the structure ofa base station embodying the transmission power control of theinvention.

[0027]FIG. 11 is a diagram showing a second example of the structure ofa mobile terminal embodying the transmission power control of theinvention.

[0028]FIG. 12 is a diagram illustrating an uplink traffic channeltransmission power control method of a conventional portable telephonesystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029]FIG. 1 shows the structure of a mobile communication networkapplied to the present invention. A public switched telephone network(PSTN) 200 is connected with a fixed terminal 201 such as a telephoneand a mobile communication network 202. The mobile communication network202 is connected with a plurality of base stations 203 a, 203 b, . . . .Each base station 203 communicates with mobile terminals 204 a, 204 b, .. . in its service area (cell) via radio channels 205.

[0030] In the following, the invention will be detailed by applying itto a CDMA packet communication system using reservation based accesscontrol shown in FIG. 2.

[0031] In the CDMA packet communication system using reservation basedaccess control, channels shared by a plurality of mobile terminals inthe service area include a reservation channel 1 (uplink channel), ananswer channel 2 (downlink channel) and a pilot channel 8 (downlinkchannel). The pilot channel 8 is a channel used for transmitting a pilotsignal 9 as a reference signal to each mobile terminal.

[0032] A mobile terminal having a data transmission request transmits areservation packet 4 at a desired timing by using the reservationchannel 1. The base station performs scheduling of received reservationpackets. The base station selects (schedules) a channel and a time slot(a time slot 7 is defined in an uplink traffic channel 3) via which eachmobile terminal can transmit data, from a plurality of uplink trafficchannels 3. In order to transmit the scheduling results to each mobileterminal, the base station generates an answer packet 5 corresponding tothe reservation packet. The generated answer packet 5 is transmitted tothe corresponding mobile terminal in the area by using the answerchannel 2. The mobile terminal identifies the answer packet destined toit from received answer packets 5, and transmits a data packet by usingthe uplink traffic channel and time slot designated by the base station.

[0033] In the example shown in FIG. 2, the mobile terminal transmittedthe reservation packet 4 a receives the answer packet 5 a transmitted toit, selectively from answer packets transmitted from the base station,and transmits a data packet 6 a by using the time slot 7 a of thetraffic channel 3 a designated in the received answer packet 5 a.

[0034] With reference to FIGS. 3 to 9, a first embodiment will bedescribed which realizes a method of controlling the transmission powerof an uplink channel.

[0035]FIG. 3 shows an example of the structure of a base station. Asignal received by an antenna 30 is input via a circulator 31 to areception radio module 32. The reception radio module 32 performs ahigh/middle frequency reception process to demodulate a signal in acarrier frequency band into a baseband signal. Since the received signalhas a plurality of multiplexed channel signals, it is input to anacquisition/despread circuit (33, 42 a-42 n) to be spectrum despread.

[0036] A reservation channel output from the reservation channelacquisition/despread circuit 33 is supplied via a signal line 50 to adetector 35 whereat it is detected and then supplied to a decoder 36whereat an error correction decode process such as Viterbi decoding isperformed. A packet interpretation unit 37 interprets the decodedreservation packet to obtain a terminal ID of the mobile terminal whichtransmitted the reservation packet and the reservation contents such astransmission data, and transfers the reservation contents to an answerpacket generator unit 38.

[0037] The reservation packet is also input via a signal line 51 to aunit 39 for measuring the received level of the reservation channel.This unit 39 measures a signal to noise power ratio (SN ratio) of thereservation packet. The measurement result of the received level iscompared with a reference reception level by an initial transmissionpower control signal generator 40. In accordance with this comparisonresult, a transmission power control signal is generated whichdesignates a transmission power when the mobile terminal startstransmitting a data packet. The generated transmission power controlsignal is input to an answer packet generator 38.

[0038] In accordance with the reservation contents interpreted by thepacket interpretation unit 37 and the transmission power control signalgenerated by the initial transmission power control signal generator 40,the answer packet generator 38 generates an answer packet. An example ofthe structure of an answer packet is shown in FIG. 4. A mobile terminalID is an ID of a mobile terminal which transmitted a reservation packet.This ID is used as a destination of the answer packet. An allocatedchannel 101 and an allocated slot number 102 indicate an uplink trafficchannel and a time slot to be used by the mobile terminal and aredesignated by the answer packet generator 38. An initial transmissionpower 103 indicates a transmission power when the mobile terminal startstransmitting data and is designated by the transmission power controlsignal input from the initial transmission power control signalgenerator 40. This initial transmission power control signal maydesignate an increase/decrease relative to the transmission power whenthe reservation packet was transmitted, or may be an absolute value(increased/decreased value) of the transmission power, whichever of themis determined by the system. A CRC (Cyclic Redundancy Check) 104 is acode added to the answer packet for error detection/correction.

[0039] The answer packet generated in the above manner is input to acoder 47 whereat an error correction coding such as convolutional codingis performed. The coded answer packet is input to a unit 41 forinserting a traffic channel transmission power control signal.

[0040] The other acquisition/despread circuits 42 a to 42 n provided fora plurality of uplink traffic channels each output a data packettransmitted via each uplink traffic channel. The data packet of eachchannel is supplied via a signal line 52 to a detector 43 a-43 n and adecoder 44 a to 44 n to be detected and decoded, and the reception datais output from a signal line 54.

[0041] The data packet is also supplied via a signal line 53 to a unit45 for measuring the received level of the traffic channel. Thestructure of this unit 45 is shown in FIG. 5. The received levelmeasurement units 45 a to 45 n corresponding to the uplink trafficchannels 53 a to 53 n measure the received level such as an SN ratio.

[0042] The received level measurement result of each traffic channel isinput to a traffic channel transmission power control signal generator46. The structure of the generator 46 is shown in FIG. 6. Each of thetransmission power control signal generators 46 a to 46 n provided foreach uplink traffic channel compares the received level with a targetreception level, and generates a transmission power control signal formaking the mobile terminal renew the transmission power when itcontinues data transmission. Similar to the initial transmission powercontrol signal, this renewal designation transmission power controlsignal is determined by the system. The generated transmission powercontrol signal is input to the unit 41 for inserting the traffic channeltransmission power control signal.

[0043] As shown in FIG. 7, the traffic channel transmission powercontrol signal insert unit 41 inserts a common transmission powercontrol signal 111 generated by the traffic channel transmission powercontrol signal generator 46 at a predetermined interval between answerpackets 110 input from the answer packet generator 38. The commontransmission power control signal 111 is constituted of transmissionpower control signals 111 a to 111 n of respective traffic channels 1 ton.

[0044] In order to suppress a fluctuation of the received level of adata packet, the base station is required to perform a transmissionpower control of each mobile terminal at a sufficiently high occurrencefrequency. The data packet is made of several tens of bits to allowinformation of some amount to be transmitted at the same time. Incontrast, the common transmission power control signal 111 can be madeof n bits assuming the same system as IS-95. As shown in FIG. 4, theanswer packet can be made sufficiently small relative to the size of adata packet. Therefore, as in this embodiment, even if the answerchannel and the transmission power control channel are shared, thetransmission power control can be performed at a sufficiently highoccurrence frequency. If the answer packet and the common transmissionpower control signal are received by the same channel, the mobileterminal can use a common receiver both for the answer packet and commontransmission power control signal. In this manner, the circuit scale ofeach mobile terminal can be made small.

[0045] It is also possible to transmit the common transmission powercontrol signal at a transmission power larger than that of the answerpacket in order to reliably perform the transmission power control.

[0046] The answer packet and common transmission power control signalare spectrum spread by a spreader 48 for answer channel. The spectrumspread answer packet and common transmission power control signal aremultiplexed with other downlinks by an adder 58, modulated from thebaseband signal into a signal in the carrier frequency band by atransmission radio module 49, and transmitted from the antenna 30 viathe circulator 31.

[0047] An example of the structure of a mobile terminal is shown in FIG.8.

[0048] The operation of transmitting a reservation packet from a mobileterminal will be described.

[0049] A signal received by an antenna 30 is input via a circulator 61to a reception radio module 62. The reception radio module 62 performs ahigh/middle frequency reception process to demodulate a signal in thecarrier frequency band into a baseband signal. A pilot signal outputfrom an acquisition/spread circuit 150 for pilot channel is input to aunit 151 for measuring a received level. This unit 151 measures thereceived level (e.g., SN ratio) of the pilot signal. The measurementresult of the received level is input to a reservation channel gaincalculator 152 which determines the transmission power of a reservationpacket in accordance with the received level of the pilot signal.

[0050] In the mobil communication system provided with independent pilotchannels, the pilot signal is transmitted from the base station alwaysat a constant transmission power level. Therefore, if an SN ratio of thereceived pilot signal is large, it is conceivable that the mobileterminal is near at the base station so that the reservation channelgain calculator 152 calculates a small gain. Conversely, if an SN ratioof the received pilot signal is small, it is conceivable that the mobileterminal is far from the base station so that the reservation channelgain calculator 152 calculates a large gain. In order to determine thetransmission power of a reservation packet in the above manner, anothersignal different from the pilot signal may be used so long as it allowsthe mobile terminal to know the transmission power of the base station.For example, the pilot signal whose transmission power is determined bythe system or a control signal transmitted with the transmission powervalue can satisfy the above conditions.

[0051] Next, an operation will be described in which a mobile terminaltransmitted a reservation packet to the base station receives an answerpacket transmitted from the base station.

[0052] An answer packet output from the despread circuit 63 for answerchannel is detected with a detector and subjected to an errorcorrection/decode process such as Viterbi decoding. With the aboveprocesses, it becomes possible to obtain the information of an allocatedtraffic channel and an allocated time slot contained in the answerpacket. An initial transmission power holder 125 holds an initialtransmission power signal contained in the answer packet, and inputs theinitial transmission power signal to a data channel gain calculator 124which calculates a gain so that a data packet can be transmitted at atransmission power designated by the initial transmission power signal.The calculated gain is set as the gain of a variable gain amplifier 68.

[0053] The data packet transmitted from the mobile terminal is amplifiedby the variable gain amplifier 68 at the gain designated by the datachannel gain calculator 124. The amplified signal is modulated from thebaseband signal into a signal in the carrier frequency band by atransmission radio module 69 and transmitted from the antenna 60 via thecirculator 61.

[0054] Next, transmission power control while a mobile terminaltransmits a data packet to the base station will be described.

[0055] A transmission power correction unit 123 derives the commontransmission power control signal from a signal of the answer channelprocessed by the answer channel acquisition/despread circuit 63 anddetector 64. The transmission power correction unit 123 selects atransmission power control signal of the uplink traffic channel now inuse by its mobile terminal, from the common transmission power controlsignal. For example, in the example shown in FIG. 7, the mobile terminaltransmitting a data packet by using the transmission channel 1 selectsits transmission power control signal 111 a. The selected transmissionpower control signal is input to the gain calculator 124 whichcalculates a gain so that a data packet can be transmitted at atransmission power designated by the transmission control signal, andthereafter renews the gain of the variable gain amplifier 68. Theamplified signal is modulated by the transmission radio module 69 fromthe baseband signal into a signal in the carrier frequency band, andtransmitted from the antenna 60 via the circulator 61.

[0056]FIG. 9 illustrates the state of transmission power controlrealized by the above operations of the base station and a mobileterminal.

[0057] The base station inserts common transmission power controlsignals 142 a, 142 b, 142 c, . . . into a common answer channel sharedby mobile terminals in the area and transits them. The commontransmission power control signal 142 contains transmission powercontrol signals for the respective traffic channels 1 to n. Each of themobile terminals 1 to n transmitting data packets 1 to n to the basestation derives the transmission power control signal of the trafficchannel now in use by the mobile terminal, from the common transmissionpower control signals 142 a, 142 b, 142 c, . . . . In accordance withthe derived transmission power control signal, the mobile terminalchanges the transmission power of the data packet.

[0058] In the state shown in FIG. 9, the width of a data packet is drawnto correspond to the receive level of the data packet at the basestation. For example, in the uplink traffic channel 1, the mobileterminal controls the transmission power such that the transmissionpowers are increased, reduced, and increased in response to thereception of the common transmission power control signals 142 a, 142 b,and 142 c.

[0059] While a data packet is not transmitted by a mobile station, thetransmission power control signal is neglected. The transmission powercontrol signal is also neglected if it is received before a lapse time(called “control delay time”) necessary for measuring the received levelof a data packet at the base station after the mobile terminaltransmitted the data packet. The reason for this is a possibility thatthe transmission power control information received before the lapse ofthe control delay time may be a transmission power control informationof a data packet transmitted by another mobile terminal and erroneouscontrol to be made.

[0060] With the above operations, it becomes possible for the basestation to perform transmission power control of the uplink trafficchannels 1 to n by using the common control channel shared by mobileterminals.

[0061] This first embodiment has the structure suitable for datacommunication, particularly for one way data communication. Two way datacommunication is performed in some case. In this case, the transmissionpower control signal may be contained in data of a downlink trafficchannel. In the following, a mobile communication system of the secondembodiment will be described which is suitable for two way communicationand has a simple circuit structure, particularly of a mobile terminal.

[0062]FIG. 10 shows an example of the structure of a base stationaccording to the second embodiment.

[0063] In FIG. 10, like constituent elements to those of the basestation of the first embodiment are represented by identical referencenumerals. The operation of the base station when a reservation packet isreceived is similar to the first embodiment.

[0064] The base station operates in the manner similar to the firstembodiment to decode a received data packet and obtain reception datafrom the signal line 54. The unit 45 for measuring the received level ofa traffic channel and the traffic channel transmission power controlsignal generator 46 generate transmission power control signals ofrespective uplink traffic channels.

[0065] In the second embodiment, if a mobile terminal transmits andreceives a data packet to and from the base station by using an uplinktraffic channel i and a downlink traffic channel k, the base stationinputs the transmission power control signal of the uplink trafficchannel i to the traffic channel transmission power control signalinsert unit 59 of the downlink traffic channel k to insert thetransmission power control signal into the data packet.

[0066] The operation will be detailed by taking as an example the casewherein the base station transmits a data packet by using a downlinktraffic channel n to a mobile terminal which transmits a data packet tothe base station by using an uplink traffic channel 1. In this case, thetransmission power control signal of the uplink traffic channel 1generated by the traffic channel transmission power control signalgenerator 46 is input to a traffic channel transmission power controlsignal insert unit 59 n of the downlink transmission channel n. Thetraffic channel transmission power control signal insert unit 59 ninserts the transmission power control signal in the data packet. Thisdata packet is spectrum spread by the spreader 57 n and multiplexed withother channel signals by the adder 58. The multiplexed signal ismodulated by the transmission radio module 49 from the baseband signalinto a signal of the carrier frequency band, and transmitted from theantenna 30 via the circulator 31.

[0067] An example of a mobile terminal of the second embodiment is shownin FIG. 11.

[0068] In FIG. 11, like constituent elements to those of the mobileterminal of the first embodiment shown in FIG. 8 are represented byidentical reference numerals. A switch 70 is connected to 70 a toperform similar operations to the first embodiment, if the mobileterminal transmits a reservation packet, receives an answer packettransmitted from the base station, or only transmits a data packet tothe base station (one way communication).

[0069] Next, the operation (two way communication) will be described inwhich a mobile terminal transmits and receives a data packet to and fromthe base station. In this case, the switch 10 is turned to 70 b side.

[0070] A data packet is received via the antenna 60, circulator 61 andreception radio module 62, and subjected to a reception process by thetraffic channel acquisition/despread circuit 63 b and detector 64. Thedata packet output from the detector is subjected to errorcorrection/decoding by the decoder 65 to obtain reception data from thesignal line 66. The data packet is also input to the transmission powercorrection unit 123 which derives the transmission power control signalinserted in the data packet and inputs it to the traffic channel gaincalculator 124. The traffic channel gain calculator 124 calculates again of the variable gain amplifier 68 to renew the gain, similar to thefirst embodiment.

[0071] With the base station and mobile terminals having the abovestructures and operating in the above manner, it becomes possible for amobile terminal to perform transmission/reception of a data packetto/from the base station and reception of transmission power control bythe base station, by using either the answer channel or traffic channel.Therefore, it is sufficient if only the mobile terminal has one set of adetector and a decoder, and so the circuit scale of the mobile terminalcan be prevented from becoming large.

[0072] In the above embodiments, the invention has been applied to amobile communication system of a reservation based access control schemein which a base station transmits a transmission power control signal toeach mobile terminal by using an answer channel. The invention is alsoapplicable to a channel other than the answer channel if it is a commonchannel shared by mobile terminals. Namely, if a system uses a commonchannel shared by mobile terminals, the base station can performtransmission power control of a plurality of mobile terminals bytransmitting transmission power control signals via the single commonchannel. Obviously, a channel dedicated to transmission power controlmay be provided to perform transmission power control of mobileterminals by transmitting transmission power control signals from thebase station by using this dedicated channel.

[0073] While the present invention has been described above inconjunction with the preferred embodiments, one of ordinary skill in theart would be enabled by this disclosure to make various modifications tothis embodiment and still be within the scope nd spirit of the inventionas defined in the appended claims.

1. A transmission power control method for a spectrum spreadingcommunication system which performs communication between a base stationand a plurality of mobile terminals by spectrum spreading, wherein: saidbase station measures the reception level of a signal transmitted fromeach of said plurality of mobile terminals, generates a transmissioncontrol signal in accordance with the reception level, and transmits acommon transmission power control signal containing transmission powercontrol signals of said plurality of mobile terminals; and each of saidplurality of mobile terminals receives said common transmission powercontrol signal, derives the transmission power control signal destinedthereto from said common transmission power control signal, and controlsthe transmission power of a signal to be transmitted to said basestation in accordance with said derived transmission power controlsignal.
 2. A transmission power control method according to claim 1,wherein said transmission power control signal is a signal indicating anincrease/decrease of said transmission power and obtained throughcomparison between said reception level and a predetermined referencereception level.
 3. A transmission power control method for a spectrumspreading communication system which performs communication between abase station and a plurality of mobile terminals by using a plurality ofchannels, wherein: said plurality of channels includes first channelsallocated to said mobile terminals for transmitting a data packet tosaid base station and a second channel used by said base station totransmit a control signal to said plurality of mobile terminals, saidsecond channel being shared by said plurality of mobile terminals; saidbase station measures the reception level of a signal received at eachof said first channels, generates a transmission control signal inaccordance with the reception level, and transmits a transmission powercontrol signal of each of said first channels to said second channel, aspart of said control signal; and each of said plurality of mobileterminals receives said transmission power control signal destinedthereto at said second channel, and controls the transmission power of asignal to be transmitted via a corresponding one of said first channelsin accordance with said received transmission power control signal.
 4. Atransmission power control method according to claim 3, wherein each ofsaid first channels is allocated to each of said plurality of mobileterminals, said base station comprises third channels for transmittingdata packets to said plurality of mobile terminals, and either a pair ofsaid first channel and said third channel or only said first channel isallocated by said base station to said plurality of mobile terminals. 5.A transmission power control method for a spectrum spreadingcommunication system which performs communication between a base stationand a plurality of mobile terminals by spectrum spreading, wherein: saidbase station measures the reception level of a signal transmitted fromeach of said plurality of mobile terminals, generates a transmissioncontrol signal in accordance with the reception level, transmits acommon transmission power control signal containing transmission powercontrol signals of mobile terminals performing one way communication,and transmits a transmission signal containing a transmission powercontrol signal of a mobile station performing two way communicationthereto; and each of said plurality of mobile terminals receives saidcommon transmission power control signal or said transmission signal,derives a transmission power control signal destined thereto from saidcommon transmission power control signal or from said transmissionsignal, and controls the transmission power of a signal to betransmitted to said base station in accordance with said derivedtransmission power control signal.
 6. A transmission power controlmethod for a spectrum spreading communication system which performscommunication between a base station and a plurality of mobileterminals, wherein: each of said plurality of mobile stations receives acontrol signal transmitted from said base station, and transmits areservation packet representative of a transmission request for a datapacket, at a transmission power corresponding to the reception level ofsaid control signal; said base station receives said reservation packet,generates an initial transmission power control signal in accordancewith the reception level of said reservation packet, and transmits saidinitial transmission power control signal to a corresponding one of saidplurality of mobile terminals, said initial transmission power controlsignal being contained in an answer packet indicating a traffic channelvia which said corresponding one transmits said data packet; and saidcorresponding one starts transmitting said data packet at a transmissionpower corresponding to said initial transmission power control signalcontained in said answer packet.
 7. A transmission power control methodaccording to claim 6, wherein: said base station measures the receptionlevel of said data packet transmitted from each of said plurality ofmobile terminals, generates a transmission power control signal inaccordance with said reception level of the data packet, and transmits acommon transmission power control signal containing the transmissionpower control signals of a plurality of traffic channels; and each ofsaid plurality of mobile terminals receives said common transmissionpower control signal, derives the transmission power control signal ofthe transmission packet via which said data packet was transmitted, fromsaid common transmission power control signal, and controls thetransmission power of another data packet in accordance with saidderived transmission power control signal.
 8. A transmission powercontrol method according to claim 7, wherein when each of said pluralityof mobile terminals receives said common transmission power controlsignal, said mobile terminal neglects said common transmission powercontrol signal if a lapse time from when said data packet starts beingtransmitted exceeds a predetermined limit time.
 9. A transmission powercontrol method according to claim 7, wherein a transmission power ofsaid control signal is known in advance at each of said plurality ofmobile terminals.
 10. A spectrum spreading communication system forperforming communication between a base station and a plurality ofmobile terminals via a plurality of channels, wherein: said plurality ofchannels include uplink traffic channels for transmitting a data packetfrom each mobile terminal to said base station, a reservation channelfor transmitting a reservation packet representative of a trafficchannel allocation request from each mobile terminal to said basestation, and an answer channel for transmitting an answer packetindicating an uplink traffic channel via which a data packet istransmitted from said base station to each mobile terminal; and a commontransmission power control signal is transmitted via said answerchannel, said common transmission power control signal containing atransmission power control signal of said uplink traffic channel.
 11. Aspectrum spreading communication system according to claim 10, wherein:said answer packet contains an ID of a corresponding mobile terminalwhich transmitted said reservation packet, information of said uplinktraffic channel allocated by said base station, and initial transmissionpower control information indicating a transmission power when said datapacket starts being transmitted; and said initial transmission powercontrol information is generated in accordance with a reception power ofsaid reservation packet at said base station.
 12. A spectrum spreadingcommunication system according to claim 10, wherein said commontransmission power control signal is inserted in said answer channel ata predetermined interval.
 13. A base station for communicating with aplurality of mobile terminals by spectrum spreading, comprising: areception circuit for receiving a data packet transmitted from each ofsaid plurality of mobile terminal; a unit for measuring the receptionlevel of said received data packet; a generator for generating atransmission power control signal in accordance with said measuredreception level of said data packet; and a transmission circuit fortransmitting a common transmission power control signal to saidplurality of mobile terminals, said common transmission power controlsignal containing said generated transmission power control signal ofeach of said plurality of mobile terminals.
 14. A base station accordingto claim 13 wherein said reception circuit includes anacquisition/despread circuit for demodulating a spectrum spread signal,and said transmission circuit includes a spreader for spectrum spreadingsaid common transmission power control signal.
 15. A base station forcommunicating with a plurality of mobile terminals by spectrumspreading, comprising: a reception circuit for receiving a data packettransmitted from each of said plurality of mobile terminal; a unit formeasuring the reception level of said received data packet; a generatorfor generating a transmission power control signal in accordance withsaid measured reception level of said data packet; and a transmissioncircuit for transmitting said generated transmission power controlsignal to said plurality of mobile terminals, as part of controlinformation.
 16. A base station for communicating with a plurality ofmobile terminals by spectrum spreading, comprising: a first receptioncircuit for receiving a reservation packet representative of atransmission request for a data packet to be transmitted from each ofsaid plurality of mobile terminals; second reception circuits forreceiving data packets transmitted from said plurality of mobileterminals; a unit for measuring the reception level of said receiveddata packet; a traffic channel transmission power control signalgenerator for generating a transmission power control signal inaccordance with said measured reception level of said data packet; and atransmission circuit for transmitting a common transmission powercontrol signal to said plurality of mobile terminals, said commontransmission power control signal containing said generated transmissionpower control signals to be transmitted to said plurality of mobileterminals.
 17. A base station according to claim 16, wherein saidtransmission circuit transmits an answer packet indicating a trafficchannel via which each mobile terminal transmits said data packet, saidanswer packet being generated after the interpretation of saidreservation packet received by said first reception circuit.
 18. A basestation according to claim 17, further comprising: a unit for measuringthe reception level of said received reservation packet; and areservation channel transmission power control signal generator forgenerating an initial transmission power control signal in accordancewith said measured reception level of said reservation packet, whereinsaid initial transmission power control signal is contained in saidanswer packet.
 19. A mobile terminal for communicating with a basestation by spectrum spreading, comprising: a reception circuit forreceiving a common transmission power control signal transmitted fromsaid base station, said common transmission power control signalcontaining transmission power control signals of a plurality of mobileterminals; a calculator for calculating a gain in accordance with atransmission power control signal destined to the mobile terminal andderived from said common transmission power control signal; and atransmission circuit for transmitting a data packet at a transmissionpower corresponding to said calculated gain.
 20. A mobile terminal forcommunicating with a base station by spectrum spreading, comprising: afirst reception circuit for receiving a common transmission powercontrol signal transmitted from said base station, said commontransmission power control signal containing transmission power controlsignals of a plurality of mobile terminals, and for receiving an answerpacket transmitted from said base station, said answer packet indicatinga traffic channel via which the mobile terminal transmits a data packet;a calculator for calculating a gain in accordance with a transmissionpower control signal destined to the mobile terminal and derived fromsaid common transmission power control signal; and a transmissioncircuit for transmitting said data packet at a transmission powercorresponding to said calculated gain via said traffic channeldesignated by said answer packet.
 21. A mobile terminal according toclaim 20, wherein said answer packet includes an initial transmissionpower control signal, said calculator calculates a gain to be used atthe start of transmission in accordance with said initial transmissionpower control signal, and said transmission circuit starts transmittingsaid data packet at a transmission power corresponding to saidcalculated gain to be used at the start of transmission.
 22. A mobileterminal according to claim 20, further comprising: a second receptioncircuit for receiving a control signal transmitted from said basestation, a transmission power of said control signal being known inadvance by the mobile terminal; a unit for measuring the reception levelof said control signa; and a reservation channel gain calculator forcalculating a reservation packet gain in accordance with the receptionlevel of said measured control signal, said reservation packet gainbeing used for transmitting a reservation packet representative of atransmission request for said data packet, wherein said transmissioncircuit transmits said reservation packet at a transmission powercorresponding to said reservation packet gain.
 23. A mobile terminal forcommunicating with a base station by spectrum spreading, comprising: afirst reception circuit for receiving a common transmission powercontrol signal transmitted from said base station, said commontransmission power control signal containing transmission power controlsignals of a plurality of mobile terminals, and for receiving an answerpacket transmitted from said base station, said answer packet indicatinga traffic channel via which the mobile terminal transmits a data packet;a second reception circuit for receiving said data packet containingsaid transmission power control signal transmitted from said basestation; a switch for switching a connection to a gain calculatorbetween said first reception circuit and said second reception circuit;and a transmission circuit for transmitting said data packet at atransmission power corresponding to the gain calculated by said gaincalculator via the traffic channel designated by said answer packet,wherein said gain calculator calculates the gain in accordance with saidcommon transmission power control signal or said transmission powercontrol signal derived from said data packet.
 24. A mobile terminalaccording to claim 23, wherein said switch connects said first receptioncircuit to said gain calculator while the mobile terminal performs oneway communication, and connects said second reception circuit to saidgain calculator while the mobile terminal performs two waycommunication.
 25. A mobile terminal according to claim 23, wherein saidanswer packet includes an initial transmission power control signal,said gain calculator calculated a gain to be used for the start oftransmission in accordance with said initial transmission power controlsignal, and said transmission circuit starts transmitting said datapacket at a transmission power corresponding to said calculated gain tobe used for the start of transmission.
 26. A mobile terminal accordingto claim 23, further comprising: a second reception circuit forreceiving a control signal transmitted from said base station, atransmission power of said control signal being known in advance by themobile terminal; a unit for measuring the reception level of saidcontrol signa; and a reservation channel gain calculator for calculatinga reservation packet gain in accordance with the reception level of saidmeasured control signal, said reservation packet gain being used fortransmitting a reservation packet representative of a transmissionrequest for said data packet, wherein said transmission circuittransmits said reservation packet at a transmission power correspondingto said reservation packet gain.